Effects of niobium addition on microstructure and properties of CPM121 powder metallurgy high-speed steel

Massive vanadium additions as hard phases in powder metallurgy high-speed steels (PM HSS) lead to higher cost and bad machinability. In this study, ultrahigh alloy PM HSS with CPM121 (10W-5Mo-4Cr-10V-9Co, wt.%) as the basic composition, was directly compacted and activation sintered with near-full density (>99.0%) using pre-oxidized and ball-mixed element and carbide powders. Niobium-alloyed steels (w(V)+w(Nb)=10 wt.%) show higher hardness and wear resistance, superior secondary-hardening ability and temper resistance. But excess niobium addition (>5 wt.%) leads to coarsened carbides and deteriorated toughness. EPMA results proved that niobium tends to distribute in MC carbides and forces element W to form M6C and WC carbides. Further, the role of rotary forging on properties of niobium-alloyed steels (S3) was researched. After rotary forging with deformation of 40%, the bending strength and fracture toughness of niobium-alloyed steels could be further improved by 20.74% and 43.86% compared with those of sample S3 without rotary forging, respectively.